The present invention relates to a new and improved construction of a controlled deflection roll, also referred to in the art as a roll with bending or sag compensation.
Generally speaking, the controlled deflection roll of the present development is of the type comprising a roll shell which is supported upon a roll support by pressure-controllable hydraulic support elements at one or a number of effective directional planes. The hydraulic support elements are distributively arranged in the direction of the lengthwise axis of the controlled deflection roll. At least two of these support elements exert different supporting forces upon the roll shell in accordance with the predetermined function, these supporting forces being controlled through the use of the same pressure.
Such type of controlled deflection roll is known to the art from German Patent Publication No. 2,325,721. With the heretofore known roll the function between the supporting forces of two support elements impinged with the same pressure is simply a proportional function, since the effective or operative surfaces, namely the piston surfaces of the hydrostaticly structured support elements possess unequal surface areas. The effect of this corresponds approximately to doubling the surface area of two individual support elements which are constructionally grouped together, and it is much less a question of the operation of the system than a question of the fabrication costs, possibly also the strength of the components as to whether there is preferred the one or the other system design. However, there is not contemplated any simplification of the control.
As soon as other relationships are desired between the forces which are to be applied by the support elements, then previously there was varied the pressure of the hydraulic fluid medium which was infed in each case to such support elements. For each separate controllable support element there was heretofore required a separate control unit, typically composed of a valve, which reduced the infeed pressure in accordance with an electrical control signal, and a signal processing circuit which was arranged forwardly of the valve.
It is self-evident that the usually desired constancy of the line force which is to be produced at a roll nip must be accomplished that much more exactly the more that the individual support elements are individually controlled. An increase in the number of the controls is however associated with both problems in space and costs. According to German Patent Publication No. 2,847,029 it is possible to provide a compromise solution in that groups of support elements are combined into zones, which in each case only have operatively correlated thereto one control unit.
The behaviour of such zone-controlled roll has been theoretically examined and discussed in the publication "Das Papier", Volume 4, page 125 et seq., 1980, and specifically for a three-zone roll which however is not used in actual practice. The author of the aforementioned article has explained the causes for the deviations arising in practice of the roll nip width from a predetermined mean or average value; the roll profile retains its configuration also in the presence of a deviating adjustment of the mean line force, whereas however the size of the deviations vary. The aforementioned profile configuration therefore possesses in characteristic fashion "null crossovers" of the nip width deviation, so that the profile assumes the known M-shape or W-shape, wherein there are present four null crossovers or throughpasses.